Various embodiments of the present invention relate to a distance measuring apparatus and to a method for calculating a distance.
Among other things, conventional distance measuring apparatuses are used, for example, for detecting the piston position of linear drives with pneumatic or hydraulic cylinders. The detection of the piston position on cylinders can be both discrete, i.e. at discrete points, and continuous, i.e. constant during operation.
Discrete determination of the piston position is generally used to report the execution or completion of a piston movement to a process control (e.g. SPS) so as to thus, for example, be able to initiate the next procedural step.
For this purpose sensors or sensor devices sensitive to magnetic fields are predominantly used which detect the magnetic field of a permanent magnet located on the cylinder piston. These types of sensor are predominantly in the form of sensors sensitive to magnetic fields and are known as Reed switches, magnetoresistive (MR), giant magnetoresistive (GMR) switches, Hall switches or magnet inductive proximity switches. The sensors used here are fitted externally on the cylinder tube of the piston cylinder. If the piston moves into the detection range of one of these sensors, the latter recognizes the presence of the cylinder piston through the cylinder tube.
If on the other hand a different position is detected, the sensor is correspondingly mechanically adjusted. For every position additionally to be detected a further sensor is consequently fitted with the associated additional material, fitting, adjustment and installation costs. This generally takes place at the customer's premises. Often the cylinder is already fitted here in a machine with poor access, and adjustment of the switch distances by mechanically shifting the externally fitted magnetic switches is no longer possible.
Furthermore, additional installation space is required for these externally fitted sensors. In order to be able to provide accessibility and robustness of the sensor, increased structural complexity is often necessary. The external fitting of the sensors is not satisfactory because the latter could be disrupted by foreign influences from the outside. The attachment of the sensors fitted on the cylinder could break, and the sensors could fall to the floor. This would result in an error being signalled and the assembly line coming to a standstill. 50% of all breakdowns with pneumatic drives happen in the aforementioned way.
Continuous piston position measurement measuring systems are normally used which function potentiometrically, magnetostrictively or according to the LVDT (Linear Variable Differential Transformer) principle. With these systems the piston position is continuously issued, predominantly as an analog voltage signal. Sensors according to the LVDT principle always use a reference path when switched on. Incremental path measurements are also known as a supplement to these systems. These systems are realized, for example, by the magnetic coding of the piston rod, and so can only be used for the relative measurement of a path. In addition, when the piston rod extends any cuttings are drawn to it, and these then destroy the seal when the piston rod is retracted. Magnetostrictive and potentiometric path measuring systems are fitted externally or within the piston rod which is to be drilled hollow. The hollow piston rod weakens the drive and the external fitting causing considerable logistical complexity.
Neither the continuous nor the discrete piston position determination can be integrated into a cylinder, or only with considerable structural complexity and the associated high costs. The considerable structural complexity arises from the fact that all of the well-established sensor principles described must be adapted to the corresponding cylinder length since they have a detection range which is too short.
Patent specification PCT/EP03/00894 describes the integration of a microwave sensor into the pneumatically/hydraulically operated cylinder chamber. The sensor in this system is positioned centrally, and this space is frequently required for pneumatic end cushioning. Additionally, in this system:                the piston is designed such that it displays the most ideal possible high frequency target        the measuring accuracy of the sensor is substantially dependent upon the adaptation of the coupling probe        the measuring principle only functions in the half of the cylinder without the piston rod        dielectric holding systems are needed for fastening the antenna        